Especially with regard to the control of robotic vehicles such as are used in war theatres and the like, it is important to be able to robustly communicate with the robotic vehicle from a base station. Presently, satellite communication systems (Satcom) are used where power levels are low and often times are not useful in communicating with terrestrial vehicles, especially those having antenna orientations that are not predictable.
For instance, as a robotic vehicle moves about terrain or for instance within a building, signals arrive at the antenna utilized by the robotic vehicle with a variety of different polarizations and directions.
If for instance the antenna utilized by the robotic vehicle is vertically polarized, then it will be insensitive to incoming signals having a horizontal polarization, and these signals, especially if they are weak, will not be detected. Likewise, if one utilized a horizontally polarized antenna, it would be insensitive to signals coming in with a vertical polarization. Of course, signals that are elliptically polarized which have components in both the vertical and horizontal directions would be non-optimally received with an antenna whose polarization did not match that of the incoming wave.
It would, therefore, be desirable to provide an antenna having a characteristic that is independent of the direction of arrival and polarization of an incoming wave. Such antennas are those exhibiting circular polarization as there will be no direction that results in polarization cancellations.
More particularly, if one were utilizing a vertical dipole on a robotic vehicle, one would have reasonable 360 degree coverage, but only for vertically polarized signals. The vertical dipole would therefore be relatively insensitive to horizontally polarized signals. In short, the dipole would not be sensitive to anything straight up.
To make matters somewhat more problematic, many antennas that are mounted on robotic vehicles have masts that are purposely flexible so that if the antenna hits an object, it will bend and not trap the antenna or stop the robot. The antenna with a flexible mast has its vertical or horizontal orientation direction altered by the flexibility of the mast which means that reliable communications cannot be established if the polarization direction of the antenna is not exactly aligned with that of the incoming signal.
In short, with a robotic vehicle as it moves through the environment, the antenna may tilt at various angles and therefore compromise communications with a base station. Further, when robotic vehicles maneuver through a building, signals can come in from various different directions due to multi-path problems. Since buildings even further attenuate satellite signals, optimum antenna orientation is a requirement if one is using anything other than a circularly polarized antenna.
Moreover, on robotic vehicles there is a requirement for miniaturization. It is not possible in most instances to provide elongated whips or antennas that are large with respect to the vehicle because of the terrain through which they operate, or because of the buildings in which they move. It is therefore important to be able to provide a miniature wide band antenna which has a circular polarization in all directions.
As described in a co-pending application entitled Orientation-Independent Antenna (ORIAN) by John T. Apostolos assigned to the assignee hereof, incorporated herein by reference and filed on even date herewith, while satisfactory performance over the majority of the bandwidth of this antenna has been achieved, lowering the VSWR, especially at the lower frequencies has been a problem. For instance, while in one embodiment of the orientation-independent circularly polarized antenna less than 2.5:1 VSWR is achievable between 245 MHz and 450 MHz, the VSWR of the antenna exceeds 2.5:1 between 225 MHz and 245 MHz. Note that at these lower frequencies a transmitter may throttle down in the face of high VSWR and may even stop transmitting, thus limiting the useful bandwidth of the antenna.